Thermal imaging or thermography is a recording process wherein images are generated by the use of thermal energy. In direct thermal imaging a visible image pattern is formed by imagewise heating of a recording material. Such thermographic materials become photothermographic when a photosensitive agent (e.g. a photosensitive silver halide) is present which after exposure to UV, visible or IR light is capable of catalyzing or participating in a process bringing about changes in colour or optical density under the influence of heat.
Examples of photothermographic materials are the so called “Dry Silver” photographic materials of the 3M Company, which are reviewed by D. A. Morgan in Chapter 2 of the “Handbook of Imaging Science”, edited by A. R. Diamond, pages 43–60, published by Marcel Dekker in 1991. The photothermographic process is based on the light sensitivity of silver halide (e.g. silver bromide) and the heat developability of organic silver salts (e.g. silver behenate). The silver halide utilized in dry silver is similar to that used in classical photography but with some significant differences: in photothermography the particle size of the silver halide is smaller and the amount of the silver halide is lower than in classical silver halide emulsion layers. It is important for high sensitivity and for realising high image contrast that the silver halide grains be dispersed in the organic silver salt and be in synergistic association with the organic silver salt particles.
The Theory of the Photographic Process Fourth Edition, edited by T. H. James, Eastman Kodak (1977), pages 149–160 discuss the sensitization of silver halide in classical silver halide emulsions. The chemical sensitization involved may be of one or more of: reduction, sulfur, gold sensitization. In such processes, chemical sensitization is obtained by adding a certain compound (“sensitizer”) to the silver halide emulsion, possibly resulting in a higher speed i.e. a higher photographic sensitivity. For sulfur-sensitization certain compounds with labile sulfur atoms, such as thiosulfate, thiourea or allylthiourea, are used.
U.S. Pat. No. 3,761,279 discloses a photothermographic element comprising a support having thereon (a) an oxidation-reduction image-forming combination comprising the reducing agent 2,2′-dihydroxy-1,1′-binaphtyl and silver behenate, (b) photosensitive silver halide, (c) a sensitising dye comprising 3-carboxymethyl-5-[(3-methyl-2-(3H)-thiazolidene)isopropylidene]rhodanine, (d) an activator-toning agent comprising phtalimide, and (d) a binder comprising polyvinylbutyral. The described combination provides a photothermographic material with improved properties such as an increased photosensitivity, faster development, higher maximum density, a more neutral tone and less background density due to post-processing print-out. The described combination can also contain a divalent metal salt image amplifier, an image stabilizer precursor and a photographic speed increasing onium halide. However, the invention was only exemplified in solvent medium, and the invention is only enabled for dispersions of silver behenate and in-situ formed silver halide in a solvent medium.
EP-A 1 132 767 discloses a photothermographic material comprising on a support a light sensitive layer containing an organic silver salt, light sensitive silver halide grains and a solvent, wherein the organic silver salt comprises tabular organic silver salt grains exhibiting an average needle ratio of not less than 1.1 and less than 10.0, the light sensitive silver halide grains having been subjected to chemical sensitization. An enhanced sensitivity can be obtained by chemical sensitization of the silver halide using an organic sensitizer containing a chalcogen atom, but this process is only exemplified in an organic solvent environment.
WO 98/45754 discloses a method for chemically sensitizing silver halide grains in a photothermographic emulsion comprising the steps of: (a) providing a photothermographic emulsion comprising silver halide grains and a non-photosensitive silver source; (b) providing a sulfur-containing compound positioned on or around the silver halide grains; and (c) sensitizing the silver halide grains by decomposing the sulfur-containing compound on or around the silver halide grains. As exemplified in this sensitizing step the sulfur-containing compound only decomposes under the influence of an oxidizing agent such as pyridinium-bromide-perbromide. Furthermore, this sensitization process is only exemplified in organic solvent media.
U.S. Pat. No. 5,922,529 discloses a photothermographic material comprising a binder, an organic silver salt, a reducing agent for silver ion, and photosensitive silver halide grains on at least one surface of a support, wherein a photosensitive layer containing the photosensitive silver halide grains further contains a compound of the formula (I) and a compound of the formula (II), and said photosensitive layer has an absorbance of 0.15 to 1.0 at an exposure wavelength,R—S(M)n   (I)wherein R is an aliphatic hydrocarbon, aryl or heterocyclic group, M is a hydrogen atom, or cation, and letter n is a number determined so as to render the molecule neutral,
wherein Z1 is a group of atoms necessary to form a 5- or 6-membered nitrogenous heterocycle, each of D and D′ is a group of atoms necessary to form an acyclic or cyclic acidic nucleus, R1 is an alkyl group, L1, L2, L3, L4, L5, L6, L7, L8, L9, and L10 each are a methine group, which may form a ring with another methine group or a ring with an auxochrome, letters n1, n2, n3, n4, and n5 each are equal to 0 or 1, M1 is an electric charge neutralizing counter ion, and letter m1 is an integer inclusive of 0 necessary to neutralize an electric charge in a molecule.
U.S. Pat. No. 4,617,257 discloses a heat developable light-sensitive material which is sufficiently stable to be stored for a long time at a predetermined sensitivity, comprising a light-sensitive silver halide spectrally sensitized with a dye represented by formula (I) or (II):
wherein R1 represents an alkyl group or a substituted alkyl group, A1 represents an alkyl group, a substituted alkyl group, a phenyl group, a substituted phenyl group, a pyridyl group, or a substituted pyridyl group, Y1 represents an oxygen atom, a sulfur atom, a selenium atom,
(wherein R3 and R4 each represents a methyl group or an ethyl group), ═N—R5 (wherein R5 represents an alkyl group or a substituted alkyl group containing not more than 5 carbon atoms, or an allyl group), or —C═CH—, Y2 represents an oxygen atom, a sulfur atom, or ═N-A2, (wherein A2 represents an alkyl or substituted alkyl group containing not more than 5 carbon atoms, an allyl group, a phenyl group, a substituted phenyl group, a pyridyl group, or a substituted pyridyl group), Z1 represents atoms forming an unsubstituted or substituted benzene or naphthalene ring; and L1 through L6 each represents a methine or substituted methine group, wherein L2 and L4 or L4 and L6 can bonded to each other to form a 5- or 6-membered ring;
wherein Y3, R7, and A3 represent the same atoms or groups as defined for Y2, R1, and A1, respectively; A4 represents an alkyl group containing from 1 to 4 carbon atoms, a halogen atom, a phenyl group, a hydroxyl group, an alkoxy group containing from 1 to 4 carbon atoms, a carboxyl group, an alkoxycarbonyl group, an alkylsulfamoyl group, an alkylcarbamoyl group, an acyl group, a cyano group, a trifluoromethyl group, or a nitro group; and L7 through L10 each represents a methine or substituted methine group, wherein the substituent is an alkyl group, a substituted alkyl group, an aryl group, or an alkoxy group, wherein said material further comprises at least one of a compound being capable of releasing a mobile dye corresponding to or inversely corresponding to the reduction of silver halide to silver at elevated temperatures and an alkali or alkali precursor.
Prior art processes for preparing photothermographic materials with high photosensitivity prepare the organic silver salt and the silver halide in aqueous media, but instead of continuing in aqueous media the organic silver salt/silver halide are separated off and dried before being dispersed in an organic solvent medium. This is inherently inefficient and also environmentally unsound as evaporation of solvent takes place during the coating process and this solvent either is vented or has to be recovered with expenditure of energy. Furthermore it involves lengthy utilization of plant during the preparation of the organic silver salt dispersion and coating requires costly plant due to the need for solvent explosion prevention measures.
For ecological and economic reasons, a process is therefore required for producing photothermographic recording materials with high photosensitivity from aqueous media.
The classical chemical sensitization procedures known from classical silver halide emulsion photography are ineffective in boosting the photosensitivity of photothermographic materials. In fact photothermographic materials are distinguished from classical silver halide emulsion materials by their significantly lower photosensitivities.
A means of boosting the photosensitivity of photothermographic materials coated from aqueous media is therefore required.